Adding #field_t and #field_t= alias methods for #field_translations
Support for using aliased field names in combination with validates_uniqueness. (Non-aliased names continue to function as before.)

Specs cover:
- aliased field names in combination with validates_uniqueness
- the presence of aliased field key in the errors hash
- aliased field in embedded documents
Replicate problem - actually getting a bson_dump error because mongoize is not called on the value
Mongoize the value when building the uniqueness query